1. Field of the Invention
This invention relates generally to a magnetic recording medium and, more particularly, to such a medium that provides improved characteristics ideally suited to record digital video signals.
2. Related Applications
U.S. Ser. No. 07/784,751, filed Oct. 30, 1991 and assigned to the Assignee of the present invention.
Description of the Prior Art
D1 format component type digital VTRs and D2 format composite type digital VTRs have been developed for use by broadcasting stations to digitize color video signals and to record the digitized signals on a recording medium, such as a magnetic tape.
In the D1 format digital VTR, a luminance signal (Y) and first and second color difference signals (U and V) are converted by analog-to-digital (A/D) conversion using sampling frequencies of 13.5 MHz and 6.75 MHz, respectively, to digitize the luminance signal and to digitize each of the color difference signals. Thereafter, the signals are suitably processed and then recorded on a magnetic tape. Since the ratio of sampling frequencies of the signal components is 4:2:2, this system is usually referred to as the 4:2:2 system.
On the other hand, in the D2 format video digital VTR, a composite video signal is sampled with a signal having a frequency 4 times higher than the frequency fsc of the usual color subcarrier signal and then A/D converted. Thereafter, the resultant signal is suitably processed and then recorded on a magnetic tape.
Since these known D1 and D2 format digital VTRs are designed for professional use, for example, in broadcasting stations, the attainment of high picture quality is given top priority in the design and construction of such VTRs, and the weight and size of the apparatus are not overly important.
In these known digital VTRs, the digital color video signal, which results from each sample being digitized into, for example, 8 bits, is recorded without being substantially compressed. As an example, when the known D1 format digital VTR A/D converts each sample into 8 bits with the frequencies noted above, the data rate representing the color video signal is 13.5 MHz.times.8+6.75 MHz.times.8+6.75 MHz.times.8 or approximately 216 Mbps (megabits per second). If the horizontal and vertical blanking intervals are ignored, the number of effective picture elements of the luminance signal per horizontal interval and the number of effective picture elements of each color difference signal per horizontal interval become 720 and 360, respectively. In the NTSC system, which has 525 lines per frame and 60 fields per second, the number of effective scanning lines for each field is 250, and the data bit rate Dv can be expressed as follows:
Dv=(720+360+360).times.8.times.250.times.60=172.8 Mbps
In the PAL system, since the number of effective scanning lines for each field is 300 and the number of fields per second is 50, it is clear that the data bit rate is the same as that in the NTSC system. If redundant data necessary for error correction and if formatting are considered, the total bit rate of picture data becomes approximately 205.8 Mbps.
Further, the amount of audio data Da to be transmitted with the digitized video signals is approximately 12.8 Mbps, while the amount of additional data Do, such as, data representing a gap, a preamble, and a postamble used in editing, is approximately 6.6 Mbps. Thus, the overall bit rate of information data to be recorded can be expressed by the following equation : EQU Dt=Dv+Da+Do EQU Dt=172.8+12.8+6.6=192.2 Mbps.
To record this amount of information data, the known D1 format digital VTR employs a segment system having a track pattern comprised of 10 tracks for each field in the NTSC system, or comprised of 12 tracks for each field in the PAL system.
In the known D1 format digital VTRs, a recording tape with a width of 19 mm is used. Typically, there are two types of recording tapes having thicknesses of 13 .mu.m. and 16 .mu.m, respectively. To house these tapes, three different sizes of cassettes are used, which are respectively characterized as being large, middle and small. The information data is recorded on such tapes in the above mentioned format with the tape area for each bit of data being approximately 20.4 .mu.m.sup.2 /bit, which corresponds to a recording density of 1/20.4 bit/.mu.m.sup.2. If the recording density is increased, an error tends to take place in the playback output data due to interference between codes or non-linearity of the electromagnetic conversion system of the head and tape. Heretofore, even if error correction encoding has been performed, the above given value of the recording density has been the limit therefor.
By putting all the above described parameters together, the playback times for the cassettes of various sizes and the two tape thicknesses, when employed in the digital VTR in the D1 format, can be tabulated as follows:
______________________________________ Size/tape thickness 13.mu.m 16.mu.m ______________________________________ Small 13 minutes 11 minutes Middle 42 minutes 34 minutes Large 94 minutes 76 minutes ______________________________________
Although the described D1 format digital VTR provides satisfactorily high picture quality for use in broadcasting stations, even if a large cassette housing a tape with a thickness of 13 .mu.m is used, the playback time is at most 1.5 hours. Such a VTR is not adequate for consumer or home use in which a playback time at least sufficient for the recording of a telecast movie is required. VTRs intended for consumer or home use with adequate recording/playback time include the .beta. system, the VHS system and the 8-mm system, each of which records and reproduces analog video signals. Although the picture quality of these analog VTRs has been improved to the point where quality is satisfactory when a video signal is simply recorded and then reproduced for viewing, the quality o the picture is significantly degraded when the recorded signal is dubbed and/or copied. Thus, when the recorded signal is dubbed several times, the picture quality will become unacceptable to viewers.
If signals of wavelengths as short as 0.5 .mu.m are recorded in tracks having a width of 5 .mu.m so as to reduce the recording area to 1.25 .mu.m.sup.2 per bit, that is, a recording density of 0.8 bit per .mu.m.sup.2, and if the recording operation includes data compression which adequately compresses the recording data without resulting in substantial distortion of the reproduced signals, then long-timed digital recording and reproducing of video signals may be possible even with magnetic tapes having a width of 8 mm or less and a length that can be readily contained in a cassette of a size not substantially larger than the cassette used with the analog 8-mm system.
However, attempts to decrease the recorded bit area to about 1.25 .mu.m.sup.2 per bit have resulted in unacceptably high bit error rates upon playback.